1. Field of the Invention
The present invention relates to a toner, and to a developer, toner container, image forming apparatus, image forming method, and process cartridge, all of which uses the toner.
2. Description of the Related Art
Conventionally, in an electrophotographic image forming apparatus or the like, an electrically or magnetically formed latent image is visualized with a toner. For example, in electrophotography, a latent electrostatic image (latent image) is formed on a photoconductor, followed by developing the latent image with the toner, to form a toner image. The toner image is typically transferred onto a transfer material (a recording medium) such as paper, followed by fixing onto the recording medium.
In the fixing image for fixing the toner image on the recording medium, a thermal fixing system, such as a heating roller fixing system or heating belt fixing system, has been generally widely used in the conventional art, because of its excellent energy efficiency.
Recently, there are increasing demands from the market for image forming apparatuses of high speed and energy saving, and therefore a toner having excellent fixing ability at the low temperature close to ordinary temperature (referred to merely as “low temperature” hereinafter) and capable of providing high quality images is desired.
A thermal fixing system, such as a heat roller fixing system and heat belt fixing system, has however not satisfied these demands, especially, energy saving.
In order to achieve low temperature fixing ability of a toner, there is, for example, a method of lowering a softening point of a binder resin.
The binder resin of a low softening point, however, reduces thermal resistance of the resulting toner, which may cause a problem of so-called “blocking” that is a phenomenon that the toner particles are fused each other especially in high temperature environments. In addition, there are also a problem that the toner is fused to and stains the interior of a developing unit and to a carrier in a developing unit (a problem of toner-spent), and a problem that the toner tends to cause filming on a surface of a photoconductor (a problem of toner-filming).
Moreover, there is disclosed a technique in which a toner contains a binder resin and a plasticizer of the binder resin to reduce a glass transition temperature Tg of the toner, which enables to perform fixing at the heating temperature lower than conventional heating temperature (see Japanese Patent Application Laid-Open (JP-A) No. 2006-330392).
However, there is a possibility that an internal temperature of a developing unit may become close to 50° C. If the Tg of the toner is reduced to about 40° C. to about 50° C. using the plasticizer as in the proposed technique, there are problems that the toner-spent and toner-filming may occur as in the case of the binder resin of low softening temperature. Accordingly, it is difficult to achieve the fixing of the toner at low temperature of about 60° C., which is close to ordinary temperature, without causing toner-spent and toner-filming, merely by adding the plasticizer to the toner. Even when the fixing temperature of the fixing unit is set to 60° C., the temperature of the toner becomes about 40° C. to about 50° C. due to heat loss to the recording medium.
Further, there is disclosed a technique in which a toner contains cyclic rubber having elasticity, and wax that is a plasticizer of the cyclic rubber (see JP-A No. 2002-221825). The disclosed art aims to achieve heat resistant storage stability and hot offset resistance of a toner as well as achieving low temperature softening of the toner.
This technique however has a problem that both toner-spent and toner-filming cannot be prevented when fixing is performed at low temperature, about 60° C.
As a technique related to an adhesive label, there is disclosed a so-called delayed tack-system adhesive label, in which an adhesive layer contains a thermoplastic resin of high Tg and a solid plasticizer at ordinary temperature, and the adhesive layer does not exhibit tackiness unless heated, and exhibits tackiness as the solid plasticizer is melted upon heating to soften the thermoplastic resin (see JP-A No. 2002-88678). It may be possible to apply this technique to achieve low temperature fixing of a toner. If the temperature at which the solid plasticizer is melted is set to about 40° C. to about 50° C., however, there is a problem that the solid plasticizer is melted in a developing unit those temperature becomes about 50° C. to thereby cause toner-spent.
Similarly to the above, there is disclosed, as a technique related to an adhesive label, a technique related to a thermosensitive delayed tack adhesive, in which an adhesive layer contains a thermoplastic resin and microcapsules each encapsulating a liquid plasticizer, and the liquid plasticizer bleeds out upon fracture of the microcapsules and the bled liquid plasticizer softens the thermoplastic resin to thereby make the adhesive layer tacky (see JP-A No. 2002-97444). It may be possible to apply this technique to achieve low temperature fixing of a toner. The state where the liquid plasticizer softens the thermoplastic resin by the fracture of the microcapsules is, however, maintained not only during the fixing step, but even after the fixing step, specifically even during discharging of paper. There is therefore a problem that the softened toner is deposited on a paper discharging unit to cause jamming of paper or pealing of the formed image, and this technique cannot achieve low temperature fixing of the toner.
As a technique using microcapsules in a toner, there is disclosed an image forming apparatus using a microcapsule toner, in which the toner contains microcapsules of large diameters, each containing, in a supporting material thereof, a few types of small-diameter microcapsules capsulated with capsule shells that are fracturable upon application of the predetermined stimuli, where the toner contains a pair of reactive materials that induced a coloring reaction upon mixing thereof, and one of the reactive material is dispersed inside the shell of each small-diameter microcapsule, and the other reactive material is dispersed outside the shell of each small-diameter microcapsule (see JP-A No. 2004-347893). The low temperature fixing is, however, not within the concept of the proposed technique, and this technique is not a technique that can realize the low temperature fixing. Moreover, this technique cannot achieve all of the low temperature fixing, and prevention of toner-spent and toner-filming, even combined with the aforementioned techniques.
As mentioned above, the related art cannot achieve low temperature fixing at 60° C. or lower while maintaining the hardness of the toner to the extent where the toner-spent and toner-filming are prevented in a developing unit at the temperature close to 50° C.
Accordingly, there have currently been demands for a toner, which excel in fixing ability with low fixing temperature (e.g., 60° C. or lower), heat resistant storage stability, and prevention of toner spent and filming of the toner.